1. Field of the Invention
The present invention relates to a method of measuring a physical quantity of a vital tissue by bringing a probe for measuring the physical quantity into contact with the vital tissue and a measuring apparatus therefor. Specifically, it relates to a non-invasive measuring method: irradiating a human body with light of a near infrared region and measuring a physical quantity in the human body such as the hematic glucose concentration or the hemal oxygen saturation using output light from the human body resulting from the light, and a measuring apparatus employed therefor. The output light from the human body includes all light such as transmitted light, scattered light or reflected light outputted from the human body after the same is irradiated with light.
2. Description of the Prior Art
A non-invasive measuring method of irradiating an organism with light and measuring hemal oxygen saturation or a blood-sugar level from resultant output light has been undertaken in the field of clinical tests. The organism is a scatterer having an internal structure complicatedly varying with portions, and hence the optical path length of the light in the organism must be kept constant in order to measure concentration such as the blood-sugar level. In the case of bringing a measuring probe into contact with a vital tissue and irradiating the same with light for measuring resultant output light, no reproducible measured value can be obtained unless conditions such as the position of the vital tissue to be in contact with the measuring probe and the orientation of the measuring probe are settled.
A non-invasive measuring apparatus, for measurements by pressing an optical measuring device against a vital tissue with a mounting apparatus, has been proposed (refer to Japanese Patent Laying-Open Gazette No. 8-215180 (1996) (cited reference)). In this measuring apparatus, a sensor is pressed to come in contact with the vital tissue vertically downward from above the vital tissue.
In the case of pressing a measuring probe against an arm or a finger of a human body for measurements, the measured portion readily varies with the posture of the human in the gravitational direction (i.e., the vertical direction). Such vertical variation of the measured portion may change the pressure of the measuring probe pressed against the measured portion, depending on the position for bringing the measuring probe into contact with the arm or finger. It is known that the intensity of scattered light from a vital tissue changes if the pressure of a measuring probe coming into contact with the vital tissue changes to cause an error in the measured value particularly in the case of measuring the blood volume or a physical quantity related to blood.
There are data obtained as to how much measured values of an organism vary with the pressure. FIGS. 1A and 1B show data obtained by pressing an optical fiber probe against a single position of a finger, irradiating the finger with light from the optical fiber probe and changing the pressure (contact pressure) of the optical fiber probe pressed against the finger for measuring the intensity of resultant reflected light FIG. 1A shows a spectrum over a wide wavenumber range, and FIG. 1B is an enlarged view of a portion (b) in FIG. 1A around 6000 cm.sup.-1.
In the data shown in FIGS. 1A and 1B, the reflected light relatively changes by 3.5% between low and high pressures.
Conversely, FIG. 2 shows a result of transmission absorption measurement of a glucose solution performed for inferring how much a light quantity changes in the case of optically measuring a blood-sugar level. For example, relative change of the light intensity corresponding to a concentration change of 10000 mg/dl is about 2.5% at a glucose absorption wavelength of 1679 nm (about 6000 cm.sup.-1). Since the variation range of glucose concentration within the physiological variation range of a human body is 400 mg/dl, it comes to that there is only relative change of light quantity of 0.1% in terms of the relative change of light absorption intensity by glucose. Namely, it comes to that this relative change of the reflected light resulting from the contact pressure is 35 times the variation range of glucose concentration.
Even if an arm or the like is so fixed that the measuring probe is pressed against a constant portion, it comes to that the measured value also changes if the contact pressure between the measured probe and the vital tissue varies with change of the vertical posture and no reproducible measurement result can be obtained.
In the apparatus described in the cited reference pressing the measuring probe vertically against the organism, change of the posture of the organism or the like appears as that of the contact pressure between the measuring probe and the vital tissue, and it is difficult to obtain a result having excellent reproducibility.